The present invention relates to chemical sensors, and more particularly to temperature control for chemical sensors to more closely maintain the temperature of the sensors at a desired steady state value.
Acoustic wave chemical sensors detect the presence of an analyte chemical by a change in the acoustic properties, such as resonant frequency, caused by absorption of the analyte chemical by a thin film coated on a piezoelectric device. These sensors are very temperature sensitive, and systems incorporating them need to provide means for controlling the device temperature. A common sensor package, such as that disclosed in NASA Tech Briefs, April 1993, encloses two sensors within a metallic case incorporating a temperature sensor, one chemical sensor being a reference sensor that is isolated from a fluid being measured and the other being a test sensor across which the fluid flows. The frequency outputs of the two chemical sensors are mixed together to produce a difference frequency. The difference frequency is a measure of the analyte chemical in the fluid flowing across the test sensor. The case is heated by a heating element controlled by a feedback loop from the temperature sensor, which maintains a constant case temperature. However since the test sensor is exposed to the environment containing the analyte chemical, the temperature of the test sensor is not necessarily equal to the temperature of the case. Also the acoustic nature of the chemical sensor makes direct attachment of the temperature sensor to the chemical sensor difficult.
What is needed is an apparatus and technique for accurately controlling the temperature of the sensors in the package case.